Aircraft, during flight and/or while on the ground, may encounter atmospheric conditions that cause the formation of ice on airfoils and other surfaces of the aircraft structure, including wings, stabilizers, rudder, ailerons, engine inlets, propellers, rotors, fuselage and the like. Accumulation ice, if not removed, can add excessive weight to the aircraft and alter the airfoil configuration, causing undesirable and/or dangerous flying conditions. General aviation aircraft are particularly susceptible to the detrimental consequences of ice formation because only small amounts of ice on structural members, such as wings, tail, propellers, and the like, can significantly alter flight characteristics.
Since the earliest days of flight, attempts have been made to overcome the problem of ice accumulation, and mechanical, chemical and thermal de-ice and/or anti-ice systems have been developed for use in large commercial and military aircraft. Thermal systems include those in which bleed air or hot air from one of the compressor stages of a turbine aircraft are diverted to heat the airfoil leading edges. Other thermal systems employ electrically conductive resistance heating elements, such as those contained in heating pads bonded to the leading edges of the aircraft, or on the propeller or rotor blades, or incorporated into the structural members of the aircraft. Heating pads of this type usually consist of a thermally insulating material in contact with wire or other metal heating elements dispersed throughout the insulating layer. Because heat must be transferred from the metal heating elements to the surrounding insulating areas, these heaters are inefficient insofar as the energy and time required for heat up to a required temperature, and the time required for cool-down when the current is removed. Electrical energy for the heating elements is derived from a generating source driven by one or more of the aircraft engines or an auxiliary power unit. The electrical energy is continuously supplied to provide enough heat to prevent the formation of ice, or intermittently supplied to loosen accumulating ice. However, such systems are only usable where sufficient wattage is available to raise and/or maintain the temperature of the airfoil surface above the freezing point at typical aircraft speeds in icing conditions.
Electrothermal anti-ice and de-ice systems are classified as either evaporative or "running wet". Anti-ice evaporative systems supply enough heat to evaporate substantially all water droplets impinging upon the heated surface. The running wet de-icing systems, however, provide only enough heat to prevent freezing of the water droplets. The water then flows aft of the heated surface where it freezes, resulting in what is commonly known as runback ice. In "zoned" de-icing systems, runback ice is removed periodically by rapid application of sufficient heat to melt and loosen the ice bonded at the surface-ice interface; the bulk of the ice is then removed by aerodynamic or centrifugal forces.
In many heating pads used for electrothermal zoned de-icing systems, metal heating elements are configured as serpentine ribbons that form interconnected conductive segments. Because of the low electrical resistivity of metal heating elements, such as copper, aluminum, and the like, the serpentine configuration is designed to provide sufficient length to the element to achieve a high enough resistance to generate energy. Each ribbon is individually electrically energized by a pair of contacts, one on each end of the ribbon, and a current is transmitted through the ribbon by establishing a voltage differential between its corresponding pair of contacts, resulting in heating of the element. Heating pads such as these are described in U.S. Pat. Nos. 5,475,204 and 5,657,951. One of the problems described in association with zoned de-icing systems employing heating pads of this type is that cold spots tend to develop at intersegmental gaps between the electrically conductive segments and at interheater gaps between adjacent zones. Ice formed at these cold spots can be very difficult to melt without the consumption of excessive current. Further, in this type of heating pad, each metallic heating element requires its own electrical terminations or contact strips. Because they are not heated, the melting of ice on or around the contact strips can also be very difficult. Accumulation of ice at the intersegmental and interheater gaps and around the contact strips is particularly undesirable since the accumulated ice can serve as a "anchor" for additional ice formation. In an attempt to address the problem of "cold spots", an electrothermal de-icing pad described in U.S. Pat. No. 5,475,204 provides at least two heaters having conductive heating elements that are positioned relative to each other so that the marginal portions are overlapped in an attempt to eliminate gaps. However, as with other previously described electrothermal de-icing systems, these heaters have multiple heating zones containing a plurality of metallic heating elements, including a plurality of electrical terminations, requiring the use of complex control mechanisms that rely on multiple timers to control multiple zones.
As discussed above, the use of electrothermal heating pad systems is only feasible where sufficient wattage is available to raise and/or maintain the temperature of the airfoil surface above the freezing point at typical aircraft speeds in icing conditions. Because of the configuration of the metal elements in these pads, the watt densities are not uniformly distributed, resulting in substantial heating inefficiencies in terms of the average watt density provided. The power requirements for the anti-ice and/or de-ice systems using these metallic heating pads are large. Therefore, electrothermal systems that have been successfully used in large aircraft have been impractical for general aviation light aircraft, such as single engine and light twin airplanes and helicopters, because of power requirements that are in excess of the electrical power available. Moreover, auxiliary on-board power generating units for de-icing systems have not been employed in light aircraft because of the substantial weight and expense penalty that would be incurred.
Thus, there is a need for an ice protection system for all aircraft, including general aviation light aircraft, that has sufficient operating efficiency to protect aircraft structural members, such as the wings, tail structures, propeller, rotor blades, and the like, against the accumulation of ice, that is light weight, that does not interfere with aircraft flight characteristics, and that is economical. More particularly, there is a need for an efficient thermoelectric heater system that can be "zoned" to provide an effective de-ice system in which electrical energy can be intermittently or continuously supplied to provide heat sufficient to prevent the formation of ice or to loosen accumulating ice.
Recently, technology has been developed to allow a light aircraft to be fitted with a 150 ampere to 200 ampere alternator producing 40 to 60 volts, without a significant weight penalty. It is now possible that a combination of a very efficient thermoelectric heater, in terms of the watt density provided by the heater, and such an alternator, could also allow general aviation aircraft to utilize reliable electrothermal in-flight anti-ice/de-ice systems.